Add --single-log option to prism-auto

[davexparker]

Summary

• Adds --single-log FILE option to prism-auto to store PRISM output for all runs in a single shared file
• Uses -mainlog FILE:append to append each run's output, tracking the file offset before each run so test result / DD warning scanning only reads lines from that run
• Directory for the single log file is created automatically if it does not exist

Test plan

• prism-auto --single-log out.log ... — creates out.log fresh, appends each run's output
• Re-run — out.log is truncated at start (fresh log per prism-auto invocation)
• Test results and DD warnings are correctly attributed to each run (not the whole file)
• --single-log path/to/new/dir/out.log — missing directory is created automatically
• --single-log and -l/--log used together — confirm no interference

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